drm/amd/display: Fix possible underflow for displays with large vblank

[Why]
Underflow observed when using a display with a large vblank region
and low refresh rate

[How]
Simplify calculation of vblank_nom

Increase value for VBlankNomDefaultUS to 800us

Fixed a null pointer from previous commit of this change

Reviewed-by: Nicholas Kazlauskas <[e-mail]>
Acked-by: Tom Chung <[e-mail]>
Signed-off-by: Daniel Miess <[e-mail]>
Tested-by: Daniel Wheeler <[e-mail]>
Signed-off-by: Alex Deucher <[e-mail]>

drivers/gpu/drm/amd/display/dc/dml/dcn314/dcn314_fpu.c

···
 #include "dml/display_mode_vba.h"
 
 struct _vcs_dpi_ip_params_st dcn3_14_ip = {
-	.VBlankNomDefaultUS = 668,
+	.VBlankNomDefaultUS = 800,
 	.gpuvm_enable = 1,
 	.gpuvm_max_page_table_levels = 1,
 	.hostvm_enable = 1,
···
 	struct resource_context *res_ctx = &context->res_ctx;
 	struct pipe_ctx *pipe;
 	bool upscaled = false;
-	bool isFreesyncVideo = false;
+	const unsigned int max_allowed_vblank_nom = 1023;
 
 	dc_assert_fp_enabled();
 
···
 		pipe = &res_ctx->pipe_ctx[i];
 		timing = &pipe->stream->timing;
 
-		isFreesyncVideo = pipe->stream->adjust.v_total_max == pipe->stream->adjust.v_total_min;
-		isFreesyncVideo = isFreesyncVideo && pipe->stream->adjust.v_total_min > timing->v_total;
-
-		if (!isFreesyncVideo) {
-			pipes[pipe_cnt].pipe.dest.vblank_nom =
-				dcn3_14_ip.VBlankNomDefaultUS / (timing->h_total / (timing->pix_clk_100hz / 10000.0));
-		} else {
-			pipes[pipe_cnt].pipe.dest.vtotal = pipe->stream->adjust.v_total_min;
-			pipes[pipe_cnt].pipe.dest.vblank_nom = timing->v_total - pipes[pipe_cnt].pipe.dest.vactive;
-		}
+		pipes[pipe_cnt].pipe.dest.vtotal = pipe->stream->adjust.v_total_min;
+		pipes[pipe_cnt].pipe.dest.vblank_nom = timing->v_total - pipes[pipe_cnt].pipe.dest.vactive;
+		pipes[pipe_cnt].pipe.dest.vblank_nom = min(pipes[pipe_cnt].pipe.dest.vblank_nom, dcn3_14_ip.VBlankNomDefaultUS);
+		pipes[pipe_cnt].pipe.dest.vblank_nom = max(pipes[pipe_cnt].pipe.dest.vblank_nom, timing->v_sync_width);
+		pipes[pipe_cnt].pipe.dest.vblank_nom = min(pipes[pipe_cnt].pipe.dest.vblank_nom, max_allowed_vblank_nom);
 
 		if (pipe->plane_state &&
 				(pipe->plane_state->src_rect.height < pipe->plane_state->dst_rect.height ||